I’m familiar with the issue you are experiencing. The problem is in the way you created ramdisk and allocated space for it.
TL;DR; Bladebit needs direct_io access to write which you don’t have if you create ramdisk with tmpfs way.
For bladebit you need to create disk in a different way using modprobe tool.
sudo modprobe brd rd_nr=1 rd_size=$((SIZE_IN_GB * 1048576))
Now you have ramdisk on /dev/ram0
Create a file system first then mount it.
I used to ext4 as a filesystem it gave me less compute overhead while doing IO operations on ramdisk.
With 768GB of ram you can allocate up to 420GB and be in a save zone from out of memory errors.
P.s. Just keep in mind how much ram you have to not get out of memory error. Because linux may crash.